This invention relates to electrostatic spray guns, and in particular a method for reducing corona discharge in the barrel of an electrostatic spray gun.
Conventional electrostatic spray guns project fluid coating material such as paint, varnish, lacquer and the like in atomized or particulate form toward an object to be coated. The object to be coated is held at electrically ground potential and an electric charge is imparted to the coating material so that it will be elctrostatically attracted to the object. In order to assure that a high percentage of the coating material ejected from the electrostatic spray gun is deposited on the object, high charging voltages, up to 120 kv, are typically applied to the coating material.
When spraying many of the coating materials in use today, including powders, a readily ignitable atmosphere results in the area of the coating operation. Energizing the high voltage electrostatic charging circuit associated with the spray gun causes energy to be capacitively stored in the electrically conductive components of the charging system. If the gun is brought too close to any grounded object, the possibility arises that a spark will jump between the high voltage circuit in the gun and the grounded object igniting the flammable atmosphere in the coating area. Many recent improvements in electrostatic spray guns have been directed to reducing incendevity resulting from the discharge of capacitively stored electrical energy, such as disclosed, for example, in U.S. Pat. Nos. 4,182,490; 4,241,880; 4,273,293; and 4,335,851, all assigned to the same Assignee as this invention.
One means of damping discharge of electrical energy capacitively stored in the charging circuit, disclosed in the prior patents mentioned above, is the provision of a high-value resistor in the barrel of the gun. The resistor is interposed between a high voltage cable carrying the electrical charge from a source, and a high voltage lead which communicates with the nozzle assembly of the gun in which the coating material is charged. The high-value resistor effectively reduces energy capacitively stored in the gun barrel when properly insulated.
It has been found, however, that if air is present in the area of the connection between the resistor and high voltage lead, a corona discharge can occur which attacks such high voltage connection and the surrounding housing of the gun barrel. In order to eliminate air around the high voltage connection between resistor and lead, it has been the practice in the prior art to completely cover the resistor with a dielectric material such as grease. According to this method, the resistor is first encapsulated with grease and then inserted into a bore formed in the gun barrel so that it contacts the high voltage lead. In some instances, additional grease is placed in the gun barrel bore prior to insertion of the resistor. This method of eliminating air gaps around the resistor and providing an uninterrupted layer of nonconductive grease between the resistor and the gun barrel, and around the high voltage connection between the resistor and lead, has not been entirely effective.